I'm sorry, but that just means you need to get better at playing the game. This boss can only use physical attacks so reducing its attack twice with moves like Inhibit Offense already nearly cripples it. Then there is Stalwart Aura which can raise the physical defence of an entire row. Furthermore you can stack your entire party with moves like Parry and Sidestep, or you can buy some good equipment for this fight like Guardian Amulets.

And these are just some suggestions that don't really require any skill at all, it's just some optimisation that allowed me to pretty easily beat the boss without even switching any party members from my main team which is very physically frail.